1. Field of the Invention
The present invention relates to, in general, an external-control-type fan coupling device adopting a method which controls the rotation of a fan for cooling an engine in an automobile or the like in accordance with a temperature change of an external surrounding or a rotation change.
2. Description of the Related Art
Conventionally, as a fan coupling device of this type, there has been known a fan coupling device which is configured such that an inside of a hermetic housing which is formed of a non-magnetic casing and a cover which is mounted on the casing is supported on a rotary shaft body (a drive shaft) which mounts a drive disc on a distal end thereof by way of a bearing is divided into an oil reservoir chamber and a torque transmission chamber which houses the drive disc therein by a partition plate having an oil supply adjustment hole, and a valve element having magnetic property which opens or closes an oil circulation flow passage which is formed between the torque transmission chamber and the oil reservoir chamber is provided in the inside of the oil reservoir chamber, and an open/close control of the oil circulation flow passage is performed by operating the valve element using an actuator, wherein the rotational torque transmission from a drive side to a driven side is controlled by increasing or decreasing an effective contact area of oil in a torque transmission gap portion defined between the drive side and the driven side.
As this type of external-control-type fan coupling device, there has been known an external-control-type fan coupling device of a system which controls the rotation of a fan from the outside by operating an actuator inside the coupling device by exciting an electromagnetic coil fixed to an engine or vehicle body side. The structure forms a magnetic loop in which a magnetic flux generated by the excitation of the electromagnetic coil is transmitted to the valve element through a magnetic path of a magnetic body (shaft, valve element) having the high permeability, and the magnetic flux is again made to return to the electromagnetic coil, wherein a voltage is applied to the electromagnetic coil in response to an input signal from an ECU, and the valve element in the inside of the coupling device is opened or closed by a generated electromagnetic force thus controlling a flow rate of the torque transmission oil (see U.S. Pat. No. 6,443,283).
However, the above-mentioned conventional external-control-type fan coupling device has following drawbacks.
That is, in the method which operates the fan coupling device by transmitting the magnetic flux which excites the externally fixed electromagnetic coil to the valve element in the inside of the coupling device, it is necessary to form the magnetic loop in which the magnetic flux generated by the excitation of the electromagnetic coil is transmitted to the valve element through the magnetic path of a magnetic body (shaft, valve element) having the high permeability, and the magnetic flux is again made to return to the electromagnetic coil. Accordingly, the conventional fan coupling device has a drawback that there exists the restriction on a layout with respect to a positional relationship of the electromagnetic coil and the valve element, a drawback that the casing and the valve structure become complicated, a drawback that there exists a possibility of leaking of oil in incorporating the magnetic parts for constituting the magnetic loop into the inside of the coupling device, a drawback that leaking of magnetism is increased due to the elongated magnetic circuit, a drawback that the general-purpose property of the system to fan coupling devices which differ in size is insufficient and the like.